The Black Death was a catastrophe which wiped out nearly half the European population, with 20m people dying between 1348 and 1350.

But new research being carried out by a team from Liverpool University has shown that the disease may have produced an unexpected side-effect - resistance to the deadly HIV/Aids virus.

Professor Christopher Duncan and Dr Susan Scott have already caused shockwaves among historians with their claim that the Black Death was caused by a life-threatening virus, which has been lying dormant and could re-appear at any time.

The traditional view, still accepted by the majority of historians, is that the killer disease was a form of bubonic plague which was spread by fleas jumping from infected rats to humans.

Now the latest research by Prof Duncan and Dr Scott, from the university's School of Biological Sciences, has revealed that those who survived the Black Death may have inherited a mutant gene.

This gave their descendants, many generations later, increased resistance to the HIV virus.

Such a theory helps to explain, they say, why Aids has not taken hold in Europe to the same extent as it has in sub-Saharan Africa, which suffered from a different form of plague than that which ravaged the British Isles and the rest of continental Europe for three centuries.

Prof Duncan said: "We know that 10pc of the European population are genetically resistant to HIV. They do not catch the disease even after continued exposure, and it is only in Europe that this genetic mutation can be found.

"We believe it was the Black Death which caused this mutation, as people gradually formed an in-built resistance to the disease.

"As there was no Black Death in Africa, there was no resistance to the HIV infection.

"The lucky people who are resistant to HIV have benefited because their ancestors were resistant to Black Death, and they bore children who also carried the mutant gene."

Much of Prof Duncan and Dr Scott's researches have been carried out in the Peak District village of Eyam, one of the last English villages to be infected by an outbreak of the plague in 1665-66, where they have studied original parish records, wills and diaries to create a profile of the disease.

They claim the disease could not have been spread by the fleas on rats as the rodents could not have travelled far or quickly enough.

Instead, it was an infectious disease passed from person to person. If a similar kind of virus was to emerge in today's globalised society, the fatal illness would be spread quickly around the world.

The Liverpool research team say their revolutionary theory as to what caused the Black Death and made it spread like wildfire across Europe is gaining growing acceptance.

The scientists are due to present the latest findings of their research into the Aids-Black Death link in the Journal of Medical Genetics.